In recent years, cases have become common where gained popularity where a wireless LAN (hereinafter “WLAN”: Wireless Local Area Network”) is built in an office, home and so on given the economic performance of the implementation cost. A typical example of WLAN is IEEE 802.11 standardized by the IEEE (Institute of Electrical and Electronics Engineers). This standardized technique defines layers in an OSI model from a physical layer to a MAC (Medium Access Control) layer, which is a lower layer of a data link, is substitutable for Ethernet, which is a wired LAN channel and is furthermore based on a specification capable of providing also a roaming function, which is an additional function taking advantage of being wireless.
Furthermore, the above-described WLAN, Bluetooth (registered trademark) and UWB (Ultra Wideband) are used as systems capable of accessing local networks other than a carrier. WLAN is widely used for a portable information terminal such as a portable notebook personal computer provided with a WLAN function and PDA (Personal Digital Assistants). For cellular phone sets requiring lower power consumption, small power short-distance bidirectional wireless communication schemes such as Bluetooth (registered trademark) and UWB are attracting attention.
When a transmitter and a receiver are communicating at a certain specific frequency (communication channel) on a wireless network, if another pair of transmitter and receiver use the same communication channel, the band of data transmission is reduced, and therefore devices that use the communication channel later need to automatically change the channel to a free communication channel.
As a wireless communication device that changes a communication channel, for example, Patent Document 1 discloses one in which a wireless communication section is provided with a 2.4 GHz band front-end circuit and a 5 GHz band front-end circuit to handle two frequency bands of 2.4 GHz band and 5 GHz band and to thereby drastically increase the number of channels simultaneously settable by a WLAN system in the same area and reduce the possibility that the communication link may be interrupted by jamming.
FIG. 1 shows a configuration of a wireless communication apparatus on a conventional WLAN. In FIG. 1, wireless communication apparatus 10 is constructed by including antenna 11, transmission/reception changeover switch (T/R SW) 12, WLAN transmitting circuit 13, WLAN receiving circuit 14 and WLAN control circuit 15.
Transmission/reception changeover switch (T/R SW) 12 switches a SW at transmission timing and reception timing. WLAN transmitting circuit 13 transmits a signal on the WLAN. WLAN receiving circuit 14 receives a signal on the WLAN. WLAN control circuit 15 controls WLAN transmitting circuit 13 and WLAN receiving circuit 14.
Compared to a wired LAN used since before WLAN was introduced, the WLAN can move each terminal that makes up the WLAN within a communicable range and has a high degree of portability, but the boundary marking the communicable range is ambiguous. Since the boundary marking the communicable range is ambiguous, when a plurality of neighboring WLANs exist, terminals may be located within a range in which communicable ranges of the plurality of WLANs overlap with each other. These terminals mutually receive their respective waves within the mutually neighboring WLANs, which may cause a problem of jamming and reduce throughput.
[Jamming Detection Technique]
As a meted of detecting the above-described jamming, for example, Patent Document 2 discloses a technique of detecting interference based on an ID reception rate of a BSS (Basic Service Set) received from other than an AP (Access Point) with which the own station is communicating. The jamming detecting method described in Patent Document 2 detects whether or not interference has occurred between BSSs based on an outside BSSID reception rate calculation section that calculates a reception rate of frames including IDs of other BSSs which are different from an ID of the corresponding BSS among all received frames and the outside BSSID reception rate calculated by this outside BSSID reception rate calculation section.
Furthermore, Patent Document 3 discloses a technique of identifying interference based on received power of a received signal transmitted from a wireless device other than an AP with which the own station is communicating. The jamming decision method described in Patent Document 3 is provided with a function of deciding the degree of interference from an interference amount monitoring signal outputted from a wireless interface section, changing the operating channel and thereby deciding whether or not interference should be avoided.
[Jamming Avoidance Technique]
When jamming is detected, the following interference avoidance method is adopted. Patent Document 4 discloses a technique of avoiding, when jamming is detected, the jamming by changing a frequency channel. The jamming avoidance method described in Patent Document 4 measures, when selectively setting a plurality of frequency channels in a wireless network system to access a device terminal via wireless communication, radio wave levels of a plurality of frequency channels based on a signal received through a radio antenna at predetermined intervals, compares the measured radio wave levels with a predetermined threshold, decides whether or not to use each frequency channel, stores each frequency channel as statistical data of the frequency of use and changes the frequency channel based on the statistical data as required in execution of wireless communication with the above-described device terminal.
Furthermore, when a communication error occurs, fallback control is performed which reduces the transmission rate and increases the transmission distance. However, dropping the transmission rate down to a minimum rate through fallback causes the throughput to decrease drastically. The technique described in Patent Document 5 provides, when an error occurs due to interference, reference throughput at the time of interference and controls the speed mode so as to reach the throughput to secure a certain level of throughput in order to avoid a drastic drop of throughput caused by a drop of the transmission rate to a minimum rate through fallback. More specifically, when the wireless communication apparatus detects the presence/absence of interference from an interfering device which operates in the same frequency band as the frequency band to be received by the wireless communication apparatus and operates based on a standard different from the standard used for communication of the wireless communication apparatus, the wireless communication apparatus sets a throughput that serves as a reference in changing the speed mode as an interference throughput and thereby changes the speed mode.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-33676    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-109448    Patent Document 3: Japanese Patent Application Laid-Open No. 2004-357056    Patent Document 4: Japanese Patent Application Laid-Open No. 2005-333510    Patent Document 5: Japanese Patent Application Laid-Open No. 2005-278052